Why Such a Difference Between Summer 2012 and Summer 2013?

Differences in the Global Weather Pattern Between Summer 2012 and Summer 2013

Precipitation Differences:

The summer of 2013 thus far has been starkly different than what we saw in the Ohio Valley during the Summer of 2012. During the summer of 2012, the Ohio Valley saw a prolonged period of dry weather conditions that led to an exceptional drought across the region. The most impacted area was across the far western portion of the state where D4 drought conditions were noted (Figure 1). During the summer of 2013, a number of storm systems have affected much of the Ohio Valley providing the region with above normal rainfall. In fact, July 2013 was the 5th wettest July on record at Bowling Green and the 6th wettest July on record at Lexington (Table 1A). Consequently, no areas in the state of Kentucky are in a drought currently.

Temperature Differences:

As for temperatures, there was not much difference in temperatures between June 2012 and June 2013. However, stark differences were noted when comparing July 2012 to July 2013 as well as August 2012 to August 2013 (through 8/17/13). July 2012 was the warmest July on record at Louisville, and the 2nd warmest on record at Bowling Green and Lexington (Table 1B).

So, why the big differences between last summer and this summer?

In the most simple terms, the overall jet stream pattern across North America is quite different this summer than compared to the summer of 2012. This is not all that unusual as the jet stream pattern does fluctuate over a period of days to weeks. The jet stream is natural part of the atmosphere where fast moving ribbons of air move around the globe. These fast moving ribbons of air are caused by the vast temperature difference between the warm air in the tropics and the colder air up near the poles. Across North America, the jet stream travels from west to east and brings a number of weather systems through the United States. The jet stream pattern can be augmented to an extent by areas of drought that develop over parts of the country, and it can also be affected by areas of prolonged wetness. In addition, the jet stream can also be strongly influenced by anomalous areas of cold and warm water in the oceans. At times, the jet stream can become blocked due to large areas of high/low pressure that remain stationary. The blocks typically last several days, but can last several weeks. When a block occurs, the weather in those areas where the block is located can see periods of similar weather for an extended period of time.

In looking at all the datasets available, we can see that there were substantial changes in the sea-surface temperatures across the northern Pacific between last summer and this summer. In particular, sea-surface tempeatures across the Gulf of Alaska were well below normal and these cooler sea-surface temperatures extended southeastward down the California coast and then southwestward out toward Hawaii. In addition, we saw above normal sea-surface temperatures near Newfoundland and into the far northern Atlantic. The sea-surface temperature set up across the north Pacific is known as a negative PDO pattern. This type of set up results in a jet stream pattern that features a persistant upper level low across the western US with an equally large high pressure system over the eastern US. This pattern remained virtually stationary during July 2012 (Figure 2).

During the summer of 2013, the sea-surface temperatures across the northern Pacific changed quite a bit (Figure 3). The waters in the Gulf of Alaska warmed significantly and are currently well above normal. These warmer sea-surface temperatures extended southeastward down along the Washington and Oregon coasts, and also extend southwestward toward Hawaii. This pattern is known as a postive PDO pattern. The waters off of Newfound also remain above normal, but they are not as warm as they were last summer. This shift in the sea-surface temperature field has resulted in a change in the jet stream pattern across North America. The pattern has actually flipped where we now have a persistent upper high over the western US and a corresponding upper trough over the eastern US. This has resulted in the Ohio Valley seeing a number of weather systems rolling through the region bringing above normal rainfall and periods of unseasonably cool temperatures. On the other hand, the western US has seen a persistent dry and hot pattern develop.

Figure 2: Summer 2012 Sea-Surface and jet stream pattern.

Figure 3: Summer 2013 Sea-Surface and jet stream pattern.

Will The Current Pattern Continue for A While?

Given that it takes a while to change sea-surface temperatures in the ocean, large scale changes over the next few weeks are statistically low. While we here at the Louisville office focus on the weather forecast through the next 7 days, we do have tools that can give us some insight into the weather patterns for the next couple of weeks to a month. In the world of numerical weather forecasting, the forecast skill (or accuracy) drops with increasing time. On average, the accuracy of any numerical model at the day 5 or day 7 range is less than 20% correct. However, advancements in computer hardware and our understanding of the atmosphere have led to the development of more accurate climate models that can give some insight into how the large scale weather pattern will evolve over the next several weeks.

One of those tools is the Climate Forecast System (CFSv2). This model has had numerous upgrades built in over the last year and the forecast skill of the model has increased substantially. The links below are the latest trends in the model output. Essentially you can see the sequential order of forecasts made over the last 12 days. Based on the latest global data, the CFS suggests that the cooler than normal temperatures along with near normal precipitation is possible as we head into September (Temperature/Precipitation). We should note that these are model solutions and are not official forecasts, so changes in the forecast output are very likely over the next few weeks.